1. Field of the Invention
This invention relates generally to catheters used in sensing electrical activity within a patient and administering therapy, and more particularly to such catheters incorporating deposited thin film electrodes having improved flexibility and trackability within the body.
2. Discussion of the Related Art
A recognized procedure used in treating various cardiac arrhythmias involves ablating cardiac tissue at an appropriate site to block aberrant re-entrant paths. In a normal heart, the sino-atrial (SA) node located in right atrium depolarizes on a cyclic basis and a resulting electrical wave propagates to the atrio-ventricular (AV) node causing it to fire and send a further electrical wave down the bundle of HIS and there along the left and right bundle branches to produce a coordinated contraction of the ventricles. Arrhythmia, including atrial fibrillation, atrial flutter and tachycardia often results when the heart""s normal conduction system includes a re-entrant path from the ventricles to the atrium, resulting in the feedback of electrical impulses that mix with depolarizations of the SA node to deliver erratic signals to the AV node.
In addressing these arrhythmias, procedures referred to as electrophysiology mapping and ablation electro-physiological therapy are used. In carrying out such therapy, a physician will first steer a catheter having sensing/ablation electrodes on the distal end thereof through the patient""s vascular system and into a predetermined chamber of the heart where the treatment is to be carried out. The catheter is manipulated so as to place the electrodes into direct contact with the myocardial tissue that is sensed and/or to be ablated. Sensing and/or ablation can be performed on the endocardial surface or via coronary veins and arteries. The aberrant path and/or ectopic foci is first located using a xe2x80x9cmappingxe2x80x9d technique in which cardiac depolarization signals picked up by the electrodes are transmitted over electrical conductors in the lead to a suitable monitor/analyzer. When an aberrant conductive pathway or an ectopic foci is located, the physician positions the electrodes so that predetermined ones are in direct contact with the myocardial tissue to be ablated. The physician then activates a voltage generator, usually a source of RF voltage connected across a pair of electrodes, to effectively ablate and form a line of scar tissue interrupting the aberrant conductive pathway or to eliminate the ectopic foci.
Those desiring additional information relating to the use of ablation/mapping catheters for treating cardiac arrhythmias are referred to xe2x80x9cCatheter Ablation of the Atrioventricular Junction with Radiofrequency Energyxe2x80x9d by Landberg et al., Circulation, Vol. 80, No. 6, December 1989, xe2x80x9cRadiofrequency Catheter Ablation of Atrial Arrhythmiasxe2x80x9d, by Lesh et al., Circulation, Vol. 89, No. 3, Mar. 1994, and xe2x80x9cEfficacy of Radiofrequency Ablation of Atrial Tissue in Common Atrial Flutterxe2x80x9d, by Kirkorian et al., NASPE, May 1993. The use of electrode carrying catheters for ablating locations within a heart chamber is also disclosed in U.S. Pat. Nos. 4,641,649; 4,892,102; and 5,025,786.
The related art further discloses a number of different electrode configurations. In the Fleischhacker et al. U.S. Pat. No. 5,676,662, a conductive wire helix is wrapped about an elongated polymer catheter body where certain portions of the turns of the helix are uncoated so as to expose bare metal that forms the electrode surfaces. In the Swanson et al. U.S. Pat. No. 5,582,609 discrete helical windings are disposed near the distal end of an elongated flexible plastic catheter. The helical winding is intended to provide increased flexibility for allowing the catheter to better conform to the surface of the tissue to be ablated. In the Pomeranz et al. U.S. Pat. No. 5,558,073, the electrodes each comprise a metallic ring disposed about the circumference of the catheter body. A plurality of electrical conductors extend through a lumen of the catheter body to connect the metallic rings to terminals at the proximal end of the catheter body. Such ring electrodes are also shown in the Truckai et al. U.S. Pat. No. 5,487,757. In the related art arrangements, the ring electrodes are typically preformed bands of metal that are adhesively bonded, crimped or otherwise attached to the exterior surface of the catheter body, and which are connected by electrical conductors that extend through one or more lumens in the catheter body to connectors at its proximal end. These rings tend to be relatively thick and are therefore rigid. When it is considered that up to ten such ring electrodes are typically spaced about one centimeter apart along the distal end portion of the catheter body, they impact the ability of the distal end portion of the catheter to flex and conform to tissue structures to be mapped/ablated.
Leads constructed in accordance with the present invention can find use not only in mapping the endocardium and ablating tissue to treat cardiac arrhythmias, but also can be used in fabricating pacing and defibrillating leads as well as neurological leads placed along the spinal column by which electrical stimulation can be applied to the body for treating chronic pain.
It is a principal purpose of the present invention to provide an improved electrophysiology catheter that exhibits improved flexibility when contrasted with known related art mapping and ablation catheters. Rather than using a plurality of solid, preformed, tubular, metal ring electrodes, we instead deposit multiple layers of different metals as thin films so that the resulting multi-layer structure may have a thickness between 5 and 250 microns, but with between 5 and 20 microns being preferred. With such thin electrodes, they are able to readily flex and do not detract from the overall flexibility of the distal end portion of the catheter. Moreover, they can be configured in various shapes and patterns to optimize tissue contact.
Historically, thin metallic films have not been suitable for use as conductive electrodes on flexible polymers, although efforts to do so have been attempted. Past efforts have been plagued with a number of problems including: (1) being able to adhere the thin metal film coating to the polymer substrate, especially on flexure thereof; (2) lack of adhesion of the coating to itself (internal adhesion) as the coating is oxidized by air; (3) formation of cracks emanating in the film which lead to either loss of conductivity or undesirable xe2x80x9cspark gapsxe2x80x9d; (4) temperature build-up in the film resulting from internal micro-cracks which overwhelm the minimal heat capacity of a metal that is less than 1 mil thick; and (5) inherent stresses that exist in a columnar structured film as it is longitudinally stretched. Because of the aforementioned problems, persons skilled in the art of designing and producing electrical leads/catheters have stayed away from the use of thin films even though such thin film electrodes offer significant advantages, especially enhanced flexibility and improved configuration to optimize contact along an interior surface of a body cavity, and greater energy transfer because such films have no thermal capacity and, hence, no heating occurs of the film itself. As such, there is no loss of energy in the thermal heating of the thin film ring electrode.
The present invention provides a method for creating a thin metallic film electrode structure for use on electrical leads that may be used for detecting electrical activity within the body of a patient and for administering therapy. Using the method of the present invention, electrically conductive thin film coatings can comprise electrodes on mapping and/or ablation catheters or other electro-physiologic catheters without incurring the aforementioned problems and because of the thin structure of the multi-layer electrodes they readily flex without destroying the integrity of the electrode, allowing the electrodes to conform to the surface where mapping and/or ablation is to occur.
In broad terms, the invention comprises an electrical lead having an elongated, flexible, polymer lead body with a proximal end, a distal end and supporting at least one conductor extending from the proximal end to a predetermined zone located proximate the distal end of the lead body. At least one electrode is formed on the exterior surface of the lead body in that zone, the electrode being a conductive pad comprising a plurality of superimposed thin metallic film layers. The electrode can be applied to the lead body in a variety of configurations, including concentric bands, elongated, linear bands, discrete spots, etc. The electrode may, for example, have a composite thickness less than about 5 microns. Means are provided for connecting the thin film electrode to the conductor, whereby cardiac depolarization (EKG) signals or other nerve impulses picked up by the electrodes may be conveyed to an analyzer and RF voltages for administering therapy may be applied between a pair of such electrodes on the lead body, in the case of a bipolar lead, or between an electrode on the lead and a body plate or other return electrode where a monopolar lead is being used.
Without limitation, the first thin metal layer on the polymer substrate may be titanium, chromium, aluminum or nickel and the thickness of this first layer may typically be less than about 5 microns in thickness. To enhance adherence of this coating to the polymer substrate, the coating is either concurrently bombarded during its deposition by high energy ions, which serve to xe2x80x9cshot-peenxe2x80x9d the film layer into the surface of the polymer and to continuously break up the coating from an amorphous/columnar structure to a nanocrystalline structure, exhibiting over-lapping platelet regions. As used herein, the term xe2x80x9cnanocrystallinexe2x80x9d means that the metal layer is comprised of many, many minute plate-like structures (crystals). The concurrent bombardment is commonly referred to by the acronym xe2x80x9cIBADxe2x80x9d for ion beam assisted deposition. Alternatively, so-called ion beam enhanced deposition or xe2x80x9cIBEDxe2x80x9d may be used where ion beam bombardment is applied subsequent to the deposition of the metallic layer.
A second thin metal film layer or coating is subsequently deposited over the first layer and when formed of palladium or platinum of a thickness less than 1,000 angstroms, functions as a self-alloying/oxygen diffusion barrier layer. Again, the deposition preferably takes place in the presence of high energy ions to enhance interlacing between the base layer and the second layer, again providing a desired stress-free and non-columnar structure.
A third metal film layer is then applied to a predetermined thickness to function as a bulk conductive layer. It comprises platinum or silver or a similar conductive element or alloy and nominally lies in a range of between 500 angstroms and 50 microns in thickness, but with 0.5 to 5.0 microns being a preferred range. It is found that a third layer of about 2.0 to 3.0 microns in thickness is sufficient to meet the skin depths required for the RF electrical signal employed for ablation but that an upper limit of 250 microns still allows the catheter to possess a desired flexibility.
Finally, a fourth deposited thin metal film layer is applied over the third layer to act as a biocompatible, conductive layer. While various metals may be used, gold is preferred. Gold is a soft metal exhibiting very low stress. The fourth layer is nominally less than 1,000 angstroms in thickness, but may be in a range of from 500 angstroms to 50 microns.